ACTIVATION OF HEME OXYGENASE AND CONSEQUENT CARBON-MONOXIDE FORMATIONINHIBITS THE RELEASE OF ARGININE-VASOPRESSIN FROM RAT HYPOTHALAMIC EXPLANTS - MOLECULAR LINKAGE BETWEEN HEME CATABOLISM AND NEUROENDOCRINE FUNCTION

Heme oxygenase (HO)-catalyzed degradation of cellular heme moieties ge nerates biliverdin and equimolar amounts of carbon monoxide (CO), whic h has been implicated as a possible modulator of neural function. Tech nical difficulties preclude direct measurements of CO within intact ne rvous tissues; hence, alternative procedures are needed to monitor the formation and possible biologic functions of this gas. In the present study rat hypothalamic explants were found to generate 114+/-5 or 127 +/-11 pmol biliverdin/hypothalamus/1 h (n = 3) upon incubation with 1 or 10 mu M hemin, respectively. Ten micromolar zinc-protoporphyrin IX (Zn-PP-IX), a known inhibitor of HO, significantly decreased the degra dation of 10 mu M hemin from 127+/-11 to 26+/-11 pmol biliverdin/hypot halamus/1 h (n = 3; P < 0.01). Biliverdin was the principal product of HO-dependent heme degradation, as its possible conversion into biliru bin was precluded by hemin-dependent inhibition of biliverdin reductas e. Basal or hemin-supplemented hypothalamic incubations were also show n to generate sizable amounts of propentdyopents (PDPs), reflecting HO -independent degradation pathways which do not liberate CO and cannot be inhibited by Zn-PP-LX. Plotting the ratio of biliverdin to PDPs thu s provided an index of the efficiency with which hemin was degraded th rough biochemical pathways involving CO. Under the experimental condit ions of our study, the biliverdin/PDPs ratio varied from 0 to 32 or 15 %, depending on the absence or presence of 1 or 10 mu M hemin respecti vely: this suggested that the formation of CO was most efficient at 1 mu M hemin. Under these defined conditions, 1 mu M hemin was also foun d to inhibit the release of arginine vasopressin (AVP) evoked by depol arizing solutions of KCl. A series of experiments showed that the effe ct of hemin was counteracted by Zn-PP-IX, and also by tin-mesoporphyri n IX, which is even more selective in inhibiting HO; it was also atten uated in the presence of the gaseous scavenger ferrous hemoglobin. Fur thermore, the inhibition of AVP release could be reproduced by omittin g hemin and by incubating hypothalami under CO, whereas treatment with biliverdin had no effect. This suggested that the release of AVP was suppressed by HO degradation of hemin, yielding CO as a modulator of h ypothalamic function. These observations may be relevant to diseases c haracterized by inappropriate secretion of AVP and enzymatic disturban ces affecting the synthesis of heme and the formation of CO through th e HO pathway (e.g., acute intermittent porphyria or lead intoxication) . (C) 1997 Elsevier Science B.V.